New Zealand's Northland Region is built upon a basement consisting mainly of greywacke rocks, which are exposed on the eastern side of the peninsula. In-place Eocene coal measures crop out at Kamo, near Whangarei, and Oligocene limestone crops out at Hikurangi, near Whangarei. A subduction zone is believed to have existed to the northeast of Northland in early Miocene times, causing the Northland Allochthon to thrust over much of the peninsula and volcanic belts to develop on either side of the peninsula. Intra-plate basaltic volcanic activity has occurred around Kaikohe, the Bay of Islands, and Whangarei since late Miocene times. Sand dunes sourced from volcanoes further south occupy much of the western coast, and the Aupouri Peninsula joins previously separate islands to the mainland to form a large tombolo. Basement rocks As with most of New Zealand, the basement rocks of the Northland Region are mainly composed of greywacke (indurated sandstone), argillite, and chert, together with volcanic rocks, such as basalt pillow lava. The basement rocks are divided into a number of terranes, that are believed to have been combined together by subduction or strike-slip processes, by mid Cretaceous times (100 Ma). Murihiku Terrane rocks lies beneath the Northland Region on the western side, but do not crop out. The Murihiku Terrane was formed in Late Triassic to Late Jurassic times (220–145 Ma). A line of Dun Mountain-Maitai Terrane rocks pass NNW-SSE through the centre of the Northland Region, separating the Murihiku Terrane from the more easterly terranes, and produce a detectable Junction Magnetic Anomaly (JMA), but do not crop out. Caples Terrane rocks crop out to form the Waipapa Horst (Omahuta and Puketi forest area). The Caples Terrane was formed in Permian to Triassic times (300–200 Ma). Hunua Terrane rocks (part of the Waipapa Composite Terrane) crop out throughout much of eastern Northland, south of Whangaroa. The rocks are generally fine grained and highly deformed. The Hunua Terrane was formed in Triassic to Jurassic times (250–145 Ma). Mount Camel Terrane rocks crop out near Mount Camel and the Karikari Peninsula. They were formed in the early Cretaceous (131–104 Ma). Te Kuiti group coal and limestone The Te Kuiti Group Rocks overlie the basement rocks, and are present in Northland, Auckland, the Waikato, and King Country, although they have often been eroded or covered. Rocks containing coal were formed from swampland in Late Eocene times (37–34 Ma). The land sank,